The Blind Spot and Optical Illusions

Yesterday I presented some background material on optical illusions. Today, let’s look at a related topic – the blind spot.

Each of your eyes has an area on the retina which does not have any receptors. This is because this area is occupied by the optic nerve. So any image which falls on these receptors will not be seen.

To see this, close your left eye and look at the + in the image below. Move your right eye in closer to the + and then move farther away. At some point, the circle on the right will disappear. That is because it is in your blind spot.

You don’t notice your blind spot though, because your brain fills in the area with an image based on what is around it. This is a cognitive illusion. To see this, close your left eye again and look at the + in the picture below. Move closer and further away until the red X disappears. At just the right spot, it will look like the line has filled in where the X was.